Enzymes

Enzymes Biological Catalysts Control the speeds of chemical reactions in cells Speed up the rate of reaction without being changed in the reaction Enzymes are proteins Specific to the reactions they catalyse (one enzyme only affects the rate of one reaction) Help cells carry out chemical reactions: Quickly and efficiently At relatively low temperatures Metabolic Reactions All the chemical reactions that occur in cells   Organised into step-by-step sequences called metabolic pathways The enzymes for a particular pathway usually occur together, often in cell organelles E.g. Enzymes for photosynthesis occur in chloroplasts E.g. Enzymes for release of energy during respiration occur in mitochondria Anabolic Reactions: Build up complex compounds from simpler ones E.g. Making sugars from Carbon Dioxide and water in photosynthesis Catabolic Reactions: Break down complex molecules into simpler ones with a release of energy E.g. Digestion of food reserve to release energy during seed germination. Substrates and Products Substrate – substance present at the beginning of a reaction Product – substance into which the substrate is changed during the reaction Each enzyme changes a specific substrate into a specific product Enzymes and Proteins Enzymes are relatively large molecules made of proteins Enzymes are globular proteins (see previous notes) Denatured Enzymes: Denaturisation – The process by which proteins lose their 3D shape due to the application of some external stress or compound (e.g. heat, acids, bases etc.) Enzymes are formed from a chain of thousands of amino acids linked together by peptide bonds The chain folds in on itself to form a spherical, globular shape. Weaker bonds hold the chain in a three dimensional shape At high temperatures, the bonds maintaining the shape of the enzyme/ protein are broken and the molecules in the enzyme break down completely. Once an enzyme has lost its shape, it can no longer carry out its function and is denatured Active Sites and Enzyme Specificity Every enzyme has a unique 3D shape On the surface of every enzyme is a region called the active site The active site has a shape into which the smaller substrate molecule will fit. The substrate molecule breaks down to form products while attached to the active site Once the products are formed they leave the active site. How enzymes catalyse reactions: A substrate molecule collides with a molecule of its specific enzyme The substrate molecule binds to the active site of the enzyme like a key fitting a lock The enzyme-substrate complex is formed The enzyme then releases the products of the reaction The enzyme is free to form an enzyme-substrate complex with another substrate molecule Enzyme Controlled Reactions Reactions proceed because the products have less energy than the substrates. However, most substrates require an input of energy to get the reaction going, (the reaction is not spontaneous). The energy required to initiate the reaction is called the activation energy. When the substrate(s) react, they need to form a complex called the transition state before the reaction actually occurs. This transition state has a higher energy level than either the substrates or the product. Outside the body, high temperatures often supply the energy required for a reaction. This clearly would be hazardous inside the body though! Fortunately we have enzymes that provide an alternative way with a different transition state and lower activation energy.     The rate of the reaction without any external means of providing the activation energy continues at a much faster rate with an appropriate enzyme than without it. The maximum rate that any reaction can proceed at will depend, among other things, upon the number of enzyme molecules and therefore the number of active sites available. Factors affecting enzyme activity Temperature: Affects the shape of the active site For most enzymes in the human body, the optimum temperature is 37°C High temperatures denature enzymes Graph illustrating the effect of temperature on the rate of reaction: Enzymes work best at an optimum temperature   Below this, an increase in temperature provides more kinetic energy to the molecules involved. The numbers of collisions between enzyme and substrate will increase so the rate will too.   Above the optimum temperature, and the enzymes are denatured. Bonds holding the structure together will be broken and the active site loses its shape and will no longer work.   pH Affects the shape of the active site   Amylase works best at pH 7.0 – 7.5 (neutral – alkaline conditions) Pepsin enzyme which works best at pH 2 (acidic conditions) Graph illustrating the effect of temperature on the rate of reaction: Enzymes have an optimum pH If the pH changes much from the optimum, the chemical nature of the amino acids can change. This may result in a change in the bonds and so the tertiary structure may break down. The active site will be disrupted and the enzyme will be denatured.     Cofactors Most enzymes require additional help from cofactors, of which there are 2 main types: Coenzymes These are organic compounds, often containing a vitamin molecule as part of their structure. Coenzymes are not permanently bound to the enzyme but may be temporarily and loosely bound for the duration of the reaction and then move away once it is completed. Metal ions Most speed up the formation of the enzyme-substrate complex by altering the charge in the active site e.g. amylase requires chloride ions, catalase requires iron.   Inhibitors Inhibitors slow down the rate of a reaction. Sometimes this is a necessary way of making sure that the reaction does not proceed too fast, at other times, it is undesirable. Reversible inhibitors: Competitive reversible inhibitors: These molecules have a similar structure to the actual substrate and so will bind temporarily with the active site. The rate of reaction will be closer to the maximum when there is more ‘real’ substrate. Non-competitive reversible inhibitors: These molecules are not necessarily anything like the substrate in shape. They bind with the enzyme, but not at the active site. This binding does change the shape of the enzyme though, so the reaction rate decreases.  Irreversible inhibitors: These molecules bind permanently with the enzyme molecule and so effectively reduce the enzyme concentration, thus limiting the rate of reaction. Enzymes in Seed Germination Seed Germination A seed contains an embryo plant Many seeds are dormant  and can stay alive for years without growing When the conditions are right, the seed starts to germinate It takes up water and the embryo begins to grow –it develops a root and a shoot The embryo has no light and is too small to produce food by photosynthesis Seeds contain food stores containing starch which is insoluble Seeds contain the enzyme amylase which changes the starch into the sugar, maltose. Maltose is soluble and can dissolve in water in the plant and be carried to the embryo plant. Seeds also contain protease, which changes stored proteins into soluble substances. Enzymes in the seed only start working when the seed germinates. Enzymes in Biological Washing Powders Biological washing powders contain protein-digesting-enzymes These break down the proteins in stains into smaller molecules that are not coloured and easily removed by the detergent in the powder Enzymes in the Food Industry Enzymes are used in food manufacture as they are: Specific to one substrate Fairly cheap to produce Sometimes be re-usable Examples: Fructose Syrup from maize using amylase to convert starch into glucose and another enzyme to convert glucose into fructose Baking – Enzymes speed up the breakdown of starch in bread into sugar, allowing yeast to release more carbon dioxide Dairy – Enzymes help the ripening of blue mould cheeses Baby Foods – Enzymes pre-digest the baby food Fruit Juices – Enzymes remove the cloudiness from some juices (e.g. apple juice)